This application is a national stage of PCT/DE99/02028 filed Jul. 1, 1999 and based upon German national application 198 31 675.5 of Jul. 15, 1998 under the International Convention.
The present invention relates to a method for the continuous degassing of molten metals, especially molten copper, and for subsequent casting of the degassed metal whereby the metal melt is fed for degassing into a vacuum and after the degassing is transferred into a casting chamber.
The invention relates further to an apparatus for the continuous degassing of molten melts, especially molten copper, and subsequent casting of the degassed metal, with a vessel receiving the molten metal, a standpipe extending into the chamber of this vessel and having an upper end which opens into a degassing space, and an outlet for the molten metal.
In metallurgy, degassing of metal melts is known also as vacuum treatment. This includes metal melt after-treatments under greatly reduced pressures which enable dissolved gases in the metal melt, especially hydrogen, to be removed in an environment with reduced ambient pressure. The degassing which is here of interest usually subjects only a part of the liquid melt to the vacuum either by a vacuum circulation degassing or a vacuum lift degassing.
In vacuum circulation degassing, two tubular columns from an evacuated vessel extend below the melt surface in a casting ladle. A transport gas is fed into one of the two columns to produce a circulation and raise the metal melt into the vacuum vessel. The melt is there atomized and the desired reaction takes place. Through the other column, the degassed metal melt returns to the ladle. After a certain duration the entire content of the ladle has been passed through the vacuum vessel and degassed. The vacuum lift process, involves raising and lowering the vacuum vessel, whose column-like ends are immersed in the melt. Upon lowering the vacuum vessel, a part of the melt rises because of the vigorous movement into the vacuum vessel. If the vacuum vessel is then lifted, the steel flows by its own weight back into the ladle. By repeated operations partial quantities of the melt are caused to flow repeatedly into the degassing space so that after a treatment time of about 15 minutes, the ladle contents has passed a number of times through the degassing vessel and has been degassed.
DE 36 09 900 C2 describes a conventional process technology and device which utilizes this principle. With this process and device, at least two vacuum chambers are provided into which the molten metal is pumped up. One of these vacuum chambers is utilized for the degassing while the molten metal is ejected from the other vacuum chamber and can be mixed with the molten metal in the supply vessel. The two vacuum chambers alternately degas the molten metal. To maintain the molten state of the metal, the vacuum chambers are inductively heated. By means of this technology, however, only a quasi-continuous mode of operation is possible in that the bath level variations can only be held within narrow limits and within which the two vacuum chambers suck in the liquid metal melt and eject it. The drawback is not only the need to have two vacuum chambers in operation but also that the treated metal is mixed with the untreated metal since no forced circulation can be insured.
There are also known processes in which the metal melt is degassed in a separate furnace vessel optimized only for vacuum treatment. The melt is then cast. However with this process, apart from its high apparatus cost, additional time must be taken for molten metal aftertreatment which can give rise in a continuous casting system to increased production costs. Correspondingly, for the so-called casting jet degassing in which a casting jet is fed into a vacuum atmosphere the same applies.
Finally, degassing with a flushing gas is also used to create a substantial partial pressure differential for hydrogen separation. The efficiency of this process is, however, quite low.
A preferred field of application of the present process is in the production of oxygen-free copper (OF-copper). In this process, aside from low copper contents of the order of magnitude of 1 to 3 ppm, also low hydrogen contents of typically below 1 ppm can be achieved. This utilizes the principle that the hydrogen solubility in copper decreases with falling pressure and that normally hydrogen dissolved in copper can be removed under vacuum conditions from the metal without again increasing the oxygen content.
It is the object of the present invention to provide a method and an apparatus as stated at the outset which are so improved that a completely continuous mode of operation is possible, that the degassed metal does not come into contact with the untreated metal and that the apparatus and process technology costs are maintained as low as possible.
The object is achieved in a method whereby according to the invention the metal melt from a first chamber is transferred via a riser pipe [ascension tube] with an inlet opening lying beneath the bath level into a vacuum chamber serving as a degassing space and from there by gravity into a descending tube with a lower outlet opening which preferably lies below the bath level in the casting chamber and to the casting chamber. This process technique has the advantage that the metal melt which is transferred to the casting chamber is completely degassed by previous passage through the vacuum chamber. A mixing of the already degassed metal melt with an untreated metal melt is avoided.
Furthermore, only one vacuum chamber is required which is solely utilized for degassing and the lifting force produced in the first vessel suffices without any additional conveyor means to transfer the metal melt to the vacuum chamber and from it to the casting chamber. In contrast to the process of the state of the art, a continuous process is possible according to the invention.
Thus by controlling the metal melt feed into the first chamber and the discharge of the metal melt from the casting chamber, the bath surfaces in the first chamber and in the casting chamber can be set at different height levels. By means of the riser tube and the descending tube, a connection is made between the two chambers of the communicating pipe type, whereby corresponding to a height difference between the higher bath level in the first chamber and the bath surface in the casting chamber, a metal melt flow is maintained.
From the casting chamber the metal melt can be continuously or discontinuously discharged.
According to a further preferred feature, the first and second chambers adjoin one another and are subdivided in a lower region by a dam into two bath chambers. If the bath surface in the first and the second chambers lie below the upper edge of the dam, the metal melt is fed from the first chamber via the riser or ascension tube into the vacuum chamber and from there is fed via the descending tube into the casting chamber.
Should the vacuum chamber fill, for example as a result of a pumping defect or also in cases in which no degassing is desired, the bath surfaces can be so adjusted that they lie above the edge of the mentioned dam and that in the first and second chambers a common continuous bath level is formed and molten metal tends to flow directly into the casting chamber, bypassing the vacuum chamber.
To insure the flowability of the metal melt especially in the starting phase, the ascension tube and descent tube are heated.
The heating can be carried out especially with burners.
The degassing kinetics are very strongly dependent upon the temperature and for that reason in accordance with a further feature of the invention the metal melt is inductively heated and thus it is possible to control the degassing by the inductive heating of the melt.
According to a further feature of the invention, the retention time of the metal melt in the vacuum chamber is controlled by the pressure in this vacuum chamber.
From an apparatus point of view, the object of the present invention can be achieved with an apparatus in which the ascension tube extends into a first chamber which is supplied with the molten metal. A vacuum chamber for degassing is provided in the bottom of which the upper end of the ascension tube opens. The bottom has an outflow opening which is connected with a descending tube whose lower end forms an outlet opening in a second chamber configured as a casting chamber with an outlet nozzle.
The first and second chambers can be spatially connected together and can have a dam which subdivided the chambers in their lower region into two bath chambers, whereby the riser tube and the descending tube open in the different chambers below the upper dam edge. As already described previously, the molten metal which is present in the first chamber can only pass via the riser tube, the vacuum chamber and the descending tube as long as the bath levels on opposite sides of the dam lie below the upper dam edge.
This can be achieved by controlling the melt inflow into the first chamber as well as the discharge of the treated metal from the casting chamber. Upon failure of the vacuum chamber, the dam overflows so that the casting process will not be interrupted when no degassing is desired or there is a vacuum chamber failure.
Preferably, the riser tube and the descending tube are mutually parallel to another and vertical.
According to a feature of the invention, the riser tube and the descending tube are heated, especially by means of a burner.
With corresponding control or regulation it can be insured that the bath levels below or above the upper dam edge are adjustable. Preferably for temperature control of the metal melt in the inlet region an inductor is arranged with which a heating of the metal melt to a desired temperature can be insured for control of the degassing during continuous operation. To prevent undesired atmospheric influences on the casting chamber it can be hermetically sealed toward the exterior by a weir which ends below the bath level.